The properties of diamond, such as its extraordinary hardness, coefficient of friction, tensile strength and low compressibility, electrical resistivity, electrical carrier (electron and hole) mobility, high energy bandgap and saturation velocity, dielectric breakdown strength, low neutron cross-section (radiation-hardness), thermal conductivity, thermal expansion resistance, optical transmittance and refractive index, and chemical inertness allow this material to serve a vital role in a wide variety of industrial and technical applications.
The present invention relates generally to methods for the manufacture of synthetic diamond. More particularly, the invention is concerned with the physical structure and method of manufacture of a tool, which can itself subsequently be employed in the mechanosynthetic manufacture of other molecularly precise diamond structures. However, the same toolbuilding process is readily extended to other classes of tooltip molecules, handle materials, and mechanosynthetic processes and structures other than diamond.
1.1 Conventional Diamond Manufacturing
All prior art methods of manufacturing diamond are bulk processes in which the diamond crystal structure is manufactured by statistical processes. In such processes, new atoms of carbon arrive at the growing diamond crystal structure having random positions, energies, and timing. Growth extends outward from initial nucleation centers having uncontrolled size, shape, orientation and location. Existing bulk processes can be divided into three principal methods—high pressure, low pressure hydrogenic, and low pressure nonhydrogenic.
(A) In the first or high pressure bulk method of producing diamond artificially, powders of graphite, diamond, or other carbon-containing substances are subjected to high temperature and high pressure to form crystalline diamond. High pressure processes are of several types [1]:
(1) Impact Process. The starting powder is instantaneously brought under high pressure by applying impact generated by, for example, the explosion of explosives and the collision of a body accelerated to a high speed. This produces granular diamond by directly converting the starting powder material having a graphite structure into a powder composed of grains having a diamond structure. This process has the advantage that no press is required, as in the two other processes, but there is difficulty in controlling the size of the resulting diamond products. Nongraphite organic compounds can also be shock-compressed to produce diamond [2].
(2) Direct Conversion Process. The starting powder is held under a high static pressure of 13-16 GPa and a high temperature of 3,000-4,000° C. in a sealed high pressure vessel. This establishes stability conditions for diamond, so the powder material undergoes direct phase transition from graphite into diamond, through graphite decomposition and structural reorganization into diamond. In both direct conversion and flux processes, a press is widely used and enables single crystal diamonds to be grown as large as several millimeters in size.
(3) Flux Process. As in direct conversion, a static pressure and high temperature are applied to the starting material, but here fluxes such as Ni and Fe are added to allow the reaction to occur under lower pressure and temperature conditions, accelerating the atomic rearrangement which occurs during the conversion process. For example, high-purity graphite powder is heated to 1500-2000° C. under 4-6 GPa of pressure in the presence of iron catalyst, and under this extreme, but equilibrium, condition of pressure and temperature, graphite is converted to diamond: The flux becomes a saturated solution of solvated graphite, and because the pressure inside the high pressure vessel is maintained in the stability range for diamond, the solubility for graphite far exceeds that for diamond, leading to diamond precipitation and dissolution of graphite into the flux. Every year about 75 tons of diamond are produced industrially this way [14].
(B) In the second or low pressure hydrogenic bulk method of producing diamond artificially, widely known as CVD or Chemical Vapor Deposition, hydrogen (H2) gas mixed with a few percent of methane (CH4) is passed over a hot filament or through a microwave discharge, dissociating the methane molecule to form the methyl radical (CH3) and dissociating the hydrogen molecule into atomic hydrogens (H). Acetylene (C2H2) can also be used in a similar manner as a carbon source in CVD. Diamond or diamond-like carbon films can be grown by CVD epitaxially on diamond nuclei, but such films invariably contain small contaminating amounts (0.1-1%) of hydrogen which gives rise to a variety of structural, electronic and chemical defects relative to pure bulk diamond. Currently, diamond synthesis from CVD is routinely achieved by more than 10 different methods [163].
As noted by McCune and Baird [3], a diamond particle is a special cubic lattice grown from a single nucleus of four-coordinated carbon atoms. The diamond-cubic lattice consists of two interpenetrating face-centered cubic lattices, displaced by one quarter of the cube diagonal. Each carbon atom is tetrahedrally coordinated, making strong, directed sp3 bonds to its neighbors using hybrid atomic orbitals. The lattice can also be visualized as planes of six-membered saturated carbon rings stacked in an ABC ABC ABC sequence along <111> directions. Each ring is in the “chair” conformation and all carbon-carbon bonds are staggered. A lattice with hexagonal symmetry, called lonsdaleite, can be constructed with the same tetrahedral nearest neighbor configuration. In lonsdaleite, however, the planes of chairs are stacked in an AB AB AB sequence, and the carbon-carbon bonds normal to these planes are eclipsed. In simple organic molecules, the eclipsed conformation is usually less stable than the staggered because steric interactions are greater. Thermodynamically, diamond is slightly unstable with respect to crystalline graphite. At 298 K and 1 atm the free energy difference is 0.026 eV per atom, only slightly greater than kBT, where kB is the Boltzmann constant and T is the absolute temperature in degrees Kelvin.
The basic obstacle to crystallization of diamond at low pressures is the difficulty in avoiding co-deposition of graphite and/or amorphous carbon when operating in the thermodynamically stable region of graphite [3]. In general, the possibility of forming different bonding networks of carbon atoms is understandable from their ability to form different electronic configurations of the valence electrons. These bond types are classified as sp3 (tetrahedral), sp2 (planar), and sp1 (linear), and are related to the various carbon allotropes including cubic diamond and hexagonal diamond or lonsdaleite (sp3), graphite (sp2), and carbenes (sp1), respectively.
Hydrogen is generally regarded as an essential part of the reaction steps in forming diamond film during CVD, and atomic hydrogen must be present during low pressure diamond growth to: (1) stabilize the diamond surface, (2) reduce the size of the critical nucleus, (3) “dissolve” the carbon in the feedstock gas, (4) produce carbon solubility minimum, (5) generate condensable carbon radicals in the feedstock gas, (6) abstract hydrogen from hydrocarbons attached to the surface, (7) produce vacant surface sites, (8) etch (regasify) graphite, hence suppressing unwanted graphite formation, and (9) terminate carbon dangling bonds [4,6]. Both diamond and graphite are etched by atomic hydrogen, but for diamond, the deposition rate exceeds the etch rate during CVD, leading to diamond (tetrahedral sp3 bonding) growth and the suppression of graphite (planar sp2 bonding) formation. (Note that most potential atomic hydrogen substitutes such as atomic halogens etch graphite at much higher rates than atomic hydrogen [4].)
Low pressure or CVD hydrogenic metastable diamond growth processes are of several types [3-5]:
(1) Hot Filament Chemical Vapor Deposition (HFCVD). Filament deposition involves the use of a dilute (0.1-2.5%) mixture of hydrocarbon gas (typically methane) and hydrogen gas (H2) at 50-1000 torr which is introduced via a quartz tube located just above a hot tungsten filament or foil which is electrically heated to a temperature ranging from 1750-2800° C. The gas mixture dissociates at the filament surface, yielding dissociation products consisting mainly of radicals including CH3, CH2, C2H, and CH, acetylene, and atomic hydrogen, as well as unreacted CH4 and H2. A heated deposition substrate placed just below the hot tungsten filament is held in a resistance heated boat (often molybdenum) and maintained at a temperature of 500-1100° C., whereupon diamonds are condensed onto the heated substrate. Filaments of W, Ta, and Mo have been used to produce diamond. The filament is typically placed within 1 cm of the substrate surface to minimize thermalization and radical recombination, but radiation heating can produce excessive substrate temperatures leading to nonuniformity and even graphitic deposits. Withdrawing the filament slightly and biasing it negatively to pass an electron current to the substrate assists in preventing excessive radiation heating.
(2) High Frequency Plasma—Assisted Chemical Vapor Deposition (PACVD). Plasma deposition involves the addition of a plasma discharge to the foregoing filament process. The plasma discharge increases the nucleation density and growth rate, and is believed to enhance diamond film formation as opposed to discrete diamond particles. There are three basic plasma systems in common use: a microwave plasma system, a radio frequency or RF (inductively or capacitively coupled) plasma system, and a direct current or DC plasma system. The RF and microwave plasma systems use relatively complex and expensive equipment which usually requires complex tuning or matching networks to electrically couple electrical energy to the generated plasma. The diamond growth rate offered by these two systems can be quite modest, on the order of ˜1 micron/hour. Diamonds can also be grown in microwave discharges in a magnetic field, under conditions where electron cyclotron resonance is considerably modified by collisions. These “magneto-microwave” plasmas can have significantly higher densities and electron energies than isotropic plasmas and can be used to deposit diamond over large areas.
(3) Oxyacetylene Flame—Assisted Chemical Vapor Deposition. Flame deposition of diamond occurs via direct deposit from acetylene as a hydrocarbon-rich oxyacetylene flame. In this technique, conducted at atmospheric pressure, a specific part of the flame (in which both atomic hydrogen (H) and carbon dimers (C2) are present [19]) is played on a substrate on which diamond grows at rates as high as >100 microns/hour [7].
(C) In the third or low pressure nonhydrogenic bulk method of producing diamond artificially [8-17], a nonhydrogenic fullerene (e.g., C60) vapor suspended in a noble gas stream or a vapor of mixed fullerenes (e.g., C60, C70) is passed into a microwave chamber, forming a plasma in the chamber and breaking down the fullerenes into smaller fragments including isolated carbon dimer radicals (C2) [6]. (Often a small amount of H2, e.g., ˜1%, is added to the feedstock gas.) These fragments deposit onto a single-crystal silicon wafer substrate, forming a thickness of good-quality smooth nanocrystalline diamond (15 nm average grain size, range 10-30 nm crystallites [8-10]) or ultrananocrystalline diamond (UNCD) diamond films with intergranular boundaries free from graphitic contamination [9], even when examined by high resolution TEM [16] at atomic resolution [10]. Fullerenes are allotropes of carbon, containing no hydrogen, so diamonds produced from fullerene precursors are hydrogen-defect free [11]—indeed, the Ar/C60 film is close in both smoothness and hardness to a cleaved single crystal diamond sample [10]. The growth rate of diamond film is ˜1.2 microns/hour, comparable to the deposition rate observed using 1% methane in hydrogen under similar system deposition conditions [9, 10]. Diamond films can, using this process, be grown at relatively low temperatures (<500° C.) [10] as opposed to conventional diamond growth processes which require substrate temperatures of 800-1000° C.
Ab initio calculations indicate that C2 insertion into carbon-hydrogen bonds is energetically favorable with small activation barriers, and that C2 insertion into carbon-carbon bonds is also energetically favorable with low activation barriers [15]. A mechanism for growth on the diamond C(100) (2×1):H reconstructed surface with C2 has been proposed [16]. A C2 molecule impinges on the surface and inserts into a surface carbon-carbon dimer bond, after which the C2 then inserts into an adjacent carbon-carbon bond to form a new surface carbon dimer. By the same process, a second C2 molecule forms a new surface dimer on an adjacent row. Then a third C2 molecule inserts into the trough between the two new surface dimers, so that the three C2 molecules incorporated into the diamond surface form a new surface dimer row running perpendicular to the previous dimer row. This C2 growth mechanism requires no hydrogen abstraction reactions from the surface and in principle should proceed in the absence of gas phase atomic hydrogen.
The UNCD films were grown on silicon (Si) substrates polished with 100 nm diamond grit particles to enhance nucleation [16]. Deposition of UNCD on a sacrificial release layer of SiO2 substrate is very difficult because the nucleation density is 6 orders of magnitude smaller on SiO2 than on Si [18]. However, the carbon dimer growth species in the UNCD process can insert directly into either the Si or SiO2 surface, and the lack of atomic hydrogen in the UNCD fabrication process permits both a higher nucleation density and a higher renucleation rate than the conventional H2/CH4 plasma chemistry [18], so it is therefore possible to grow UNCD directly on SiO2.
Besides fullerenes, it has been proposed that “diamondoids” or polymantanes, small hydrocarbons made of one or more fused cages of adamantane (C10H16, the smallest unit cell of hydrogen-terminated crystalline diamond) could be used as the carbon source in nonhydrogenic diamond CVD [20-22]. Dahl, Carlson and Liu [22] suggest that the injection of diamondoids could facilitate growth of CVD-grown diamond film by allowing carbon atoms to be deposited at a rate of about 10-100 or more at a time, unlike conventional plasma CVD in which carbons are added to the growing film one atom at a time, possibly increasing diamond growth rates by an order of magnitude or better. However, Plaisted and Sinnott [23] used atomistic simulations to study thin-film growth via the deposition of very hot (119-204 eV/molecule; 13-17 km/sec) beams of adamantane molecules on hydrogen-terminated diamond (111) surfaces, with forces on the atoms in the simulations calculated using a many-body reactive empirical potential for hydrocarbons. During the deposition process the adamantane molecules react with one another and the surface to form hydrocarbon thin films that are primarily polymeric with the amount of adhesion depending strongly on incident energy. Despite the fact that the carbon atoms in the adamantane molecules are fully sp3 hybridized, the films contain primarily sp2 hybridized carbon with the percentage of sp2 hybridization increasing as the incident velocity goes up. However, cooler beams might allow more consistent sp3 diamond deposition, and other techniques [24] have deposited diamond-like carbon (DLC) films with a higher percentage of sp3 hybridization from adamantane.
1.2 Diamond Manufacturing Via Positional Diamond Mechanosynthesis
A new non-bulk non-statistical method of manufacturing diamond, called positional diamond mechanosynthesis, was proposed theoretically by Drexler in 1992 [32]. In this method, positionally controlled carbon deposition tools are manipulated to sub-Angstrom tolerances via SPM (Scanning Probe Microscopy) or similar atomic-resolution manipulator mechanisms to build diamond in vacuo. Each carbon deposition tool includes a tooltip molecule attached to a larger handle structure which is grasped by the atomic-resolution manipulator mechanism. One or more carbon atoms having one or more dangling bonds are relatively loosely bound to the tip of the tooltip molecule. When the tip is brought into contact with the substrate surface at a specific location and sufficient mechanical forces (compression, torsion, etc.) are applied, a stronger covalent bond is formed between the tip-bound carbon atom(s) and the surface, via the dangling bonds, than previously existed between the tip-bound carbon atom(s) and the tooltip structure. As a result, the tool may subsequently be retracted from the substrate and the tip-bound carbon atom(s) will be left behind on the substrate surface at the specific location and orientation desired. By repeating this process of positionally-constrained chemistry or mechanosynthesis, using a succession of similar tools, a large variety of molecularly precise diamond structures can be fabricated, placing one or a few atoms at a time on the growing workpiece.
Several analyses using the increasingly accurate methods of computational chemistry have confirmed the theoretical validity of the proposed process of positional diamond mechanosynthesis for hydrogen abstraction [25-33] and hydrogen donation [32, 33], in respect to the surface passivating hydrogen atoms, and carbon deposition [32-38], in respect to diamond surfaces and the body of diamond nanostructures. While positional diamond mechanosynthesis has not yet been demonstrated experimentally, early experiments [39] have demonstrated single-molecule positional covalent bond formation on surfaces via SPM, though in these cases bond formation was not purely mechanochemical but included electrochemical or other means. Mechanosynthesis of the Si(111) lattice has been studied theoretically [40, 41] and the first laboratory demonstration of nonelectrical, purely mechanical positional covalent bond formation on a silicon surface using a simple SPM tip was reported in 2003 [42]. In this demonstration, Osaka University researchers lowered a silicon AFM tip toward the silicon Si(111)-(7×7) surface and pushed down on a single atom. The focused pressure forced the atom free of its bonds to neighboring atoms, which allowed it to bind to the AFM tip. After lifting the tip and imaging the material, there was a hole where the atom had been (FIG. 1). Pressing the tip back into the vacancy redeposited the tip-bound selected single atom, this time using the pressure to break the bond with the tip. These manipulation processes were purely mechanical since neither bias voltage nor voltage pulse was applied between probe and sample [42].
Existing mechanosynthetic tools can only be used at ultralow temperatures near absolute zero, and hold the atom or molecule to be deposited only very weakly, and can be employed only very slowly (minutes or hours per mechanosynthetic operation). These tools include the simple diamond stylus [43] and other crude tools such as nanocrystalline diamond grown (a) on standard silicon [44, 48] AFM tips with a 30 nm radius [48], (b) on silicon cantilever tips [46, 47], (c) on tungsten STM tips [45], or (d) on 12 nm radius doped-diamond STM tips [49], using CVD [44-49] including HFCVD [44, 46] or PACVD [45] diamond deposition processes. There is a need for improved mechanosynthetic tools with a molecularly precise <0.3 nm tip radius that can operate at liquid nitrogen or even room temperatures, and can perform mechanosynthetic operations in seconds or even faster cycle times, and can conveniently be precisely manipulated to sub-Angstrom positional accuracy using conventional SPM instruments.
In 2002, Merkle and Freitas [36] proposed the first design for a class of precision tooltip molecules intended to positionally deposit individual carbon dimers on a growing diamond substrate via diamond mechanosynthesis (FIG. 2), and subsequent theoretical analysis [37, 38] has verified that this class of tooltip molecules should be useful for depositing carbon dimers on a dehydrogenated diamond C(110) crystal surface, for the purpose of building additional C(110) surface or other molecularly precise structures at liquid nitrogen or room temperatures. In FIG. 2, DCB6-Si refers to a configuration comprising a six-member ring dicarbon bridge (i.e., DCB6), which is substituted with Si. A general nomenclature is DCB6-X, where X=Si, Ge, Sn or Pb.
No specific proposals for attaching tooltip molecules such as the one illustrated in FIG. 2 A/B to larger tool handles, or complete tools for positional diamond mechanosynthesis, have previously been reported in the scientific, engineering or patent literature. While others have previously noted the need for a handle structure to manipulate the active mechanosynthetic tooltip [32, 33, 36, 38], this invention is the first practical description of how to manufacture and to attach tooltips to such a handle structure, and thus to manufacture a complete mechanosynthetic tool.
The present invention is not limited to a method for the manufacture of a complete tool which can be used for diamond mechanosynthesis. The same toolbuilding process is readily extended to other classes of tooltip molecules, handle materials, and mechanosynthetic processes and structures other than diamond. As examples, which in no way limit or exhaust the possible applications of this invention, the same method as described herein can be used to build complete mechanosynthetic tools and attach handles to: (1) other possible C2 dimer deposition tooltips proposed by Drexler [32] and Merkle [33, 34] for the building of molecularly precise diamond structures; (2) other possible carbon deposition tooltips, including but not limited to carbene tooltips as proposed by Drexler [32] and Merkle [33, 34] and monoradical methylene tooltips as proposed by Freitas [234], for the deposition of carbon or hydrocarbon moieties during the building of molecularly precise diamond structures, or other tooltips that may be used for the removal of individual carbon atoms, C2 dimers [38], or other hydrocarbon moieties from a growing diamond surface; (3) tooltips for the abstraction [25-33] and donation [32, 33] of hydrogen atoms, for the purpose of positional surface passivation or depassivation during the building of molecularly precise diamond structures, or during the building of molecularly precise structures other than diamond, or of other atoms similarly employed for passivation purposes; or (4) tooltips for the deposition or abstraction of atoms, dimers, or other moieties, to or from materials including, but not limited to, covalent solids other than diamond, silicon, germanium or other semiconductors, intermetallics, ceramics, and metals.